The invention relates to a power supply system and is particularly related to an X-ray unit having a power supply system and a method for determining operating parameters of a power supply system.
Switched-mode power supplies are frequently used for the power or voltage supply of electrical consumers. Such switched-mode power supplies usually convert the mains voltage into such voltages as are necessary for the operation of the respective consumers. Known switched-mode power supplies comprise an inverter or converter, which generates a switched a-c voltage from a d-c voltage. For this purpose such a converter has controllable switches. This switched a-c voltage (converter output voltage) is converted by a transformer into an a-c voltage having a suitable amplitude for the respective consumer, that is to say, it is stepped up or stepped down. If the consumer requires a d-c voltage supply, the secondary-side transformer-a-c voltage is rectified and generally stabilized by means of a smoothing capacitor.
In order to optimize the operation, switched-mode power supplies are operated as resonant arrangements. In these, a resonant capacitance together with the leakage inductance of the transformer (which can be generated by a discrete inductance) forms a series resonant circuit, which where necessary is extended by the secondary-side winding capacitance of the transformer to form a series-parallel resonant circuit. In operation, the working frequency of the converter is selected so that this is close to the intrinsic resonant frequency of the load circuit. As a result the voltage drop at the impedance of the resonant circuit is minimal.
One example of embodiment of a switched-mode power supply is the voltage supply of an X-ray tube. X-ray tubes are supplied with voltages ranging from approx. 40 kV to 150 kV and currents of up to 1.3 A. A controller adjusts the tube voltage to the required set value. As a control variable, the controller uses the activation of the converter, i.e. the switching frequency and where applicable the duty cycle. The resonant circuits provided in this application are generally of a very high quality. As a result the transient response is largely dependent on the frequency. Tolerances of the components used, such as different capacitance values of the resonant capacitor, or different values for the leakage inductance of the transformer, due also to aging, affect the resonant frequency.
For control purposes, therefore, the ability to proceed from known values in the case of determinant variables of the controlled systemxe2x80x94such as the capacitance of the resonant capacitance and the leakage inductance of the transformerxe2x80x94is of some interest. This can be ensured by the use of high-precision components, which is, however, correspondingly expensive. In addition, there is the problem of deviation in component values due to aging.
A power supply system, which is formed from a switched-mode power supply with an additional control device has already been described in the article xe2x80x9cFast estimation of unknown resonant frequencies by means of the VeCon chip setxe2x80x9d, EPE""97 Trondheim, 1997, Vol. 3, pp. 353-357. The control device has a current sensor for measuring the current on the primary side of the transformer and a control output for controlling the converter. The control device comprises a measuring device for determining the resonant frequency of the controlled resonant circuit. In the operation of the power supply system, in which the converter is operated at a switching frequency f, the path of the primary-side current is monitored by means of sample values. The resonant frequency is estimated from the sample values and the converter is adjusted according to the estimate, so that the power supply system is operated precisely at the resonant frequency.
With the control method indicated, stable operation is possible in the event of sudden fluctuations. For this purpose, however, monitoring of the primary-side current is necessary in routine operation, which gives rise to difficulties in measurement. Moreover, the xe2x80x9con-linexe2x80x9d method used is expensive, so that only very simple adjustments of the activationxe2x80x94in this case the excitation frequencyxe2x80x94are possible.
An object of the invention, therefore, is to propose a power supply system and a method for determining operating parameters thereof, and in particular an X-ray unit having such a power supply system, by means of which the necessary operating parameters can be determined with particular ease.
This object is achieved by a power supply system in accordance with one embodiment of the present invention includes a converter for the generation of a switched converter output voltage (Uw) and a transformer. The primary side of the transformer is fed by the converter and has a load output connected to its secondary side. The power supply having at least one resonant capacitance (Cr), which forms a resonant arrangement with a leakage inductance (Lr) of the transformer and/or an external inductance. A measuring device is included for determining operating parameters of the power supply system. The measuring device having a current sensor for measuring the current (ir) on the primary side of the transformer. The power supply includes means for controlling the converter wherein the measuring device is designed so that it activates the converter so that with initially unenergized components of the power supply system, a predetermined converter output voltage (Uw) is generated. One or more parameters (f0, Z0, Cr, Lr) characteristic of the resonant arrangement are determined from the measurement of the current (ir) on the primary side of the transformer.
In accordance with a method applying principles of the present invention for determining operating parameters of a power supply system having a converter for generating a converter output voltage (Uw), a transformer supplied thereby, a leakage inductance (Lr) and/or an external inductance in combination with a resonant capacitance (Cr) form a resonant arrangement, the system having a load output which is connected to the secondary side of the transformer wherein the method includes the steps of ensuring that the components of the power supply system (10) are unenergized, activating the converter (12) so that a predetermined converter output voltage (Uw) is generated at least for a brief excitation interval, measuring the current (ir) appearing on the primary side of the transformer at least during the excitation interval and determining one or more parameters characteristic of the resonant arrangement from the measurement of the current (ir).
The power supply system has a converter and a transformer supplied by the latter. The converter in turn has a voltage supply, usually a d-c voltage supply (intermediate circuit voltage) from which, by clocked switching, it generates an a-c voltage of a frequency predetermined by the activation. A converter may have two such switches, for example, converter topologies with four power switches also being usual. A further variant of a converter topology is specified, for example, in EP 884 830. In the context of the invention the precise design of the converter does not matter.
The converter output voltage feeds a transformer, which may likewise be of any construction and may have different numbers of turns, transformation ratios, numbers, tappings etcxe2x80x94depending on the design of the system. The transformer generally has a certain winding capacitance, which can play a part in the behavior of the controlled system. On the secondary side, the output voltage of the transformer is often rectified and smoothed by means of a smoothing capacitor. A resonant capacitance, which is usually connected in series on the primary side, forms a resonant arrangement together with the leakage inductance of the transformer.
Also forming part of the power supply system is a measuring device, which has a current sensor and means for activating the converter. The measuring device controls the measuring operation through the activation of the converter and performs current measurements via the current sensor. In practical implementation, the measuring device can assume various forms, generally being a circuit with a microprocessor or signal processor, to which the measured values are fed in digital form. The measuring device need not necessarily be a separate circuit; if there is already a microprocessor unit for performing a digital control, for example, the measuring device may also be designed as an additional function of this existing circuit.
The measuring device performs the measurement with the components of the power supply systems initially unenergized, i.e. when the capacitances have been discharged and there is no current flowing through the inductances. This can be ensured, for example, if the discharge behavior of the energy storage devices present in the system (such as capacitors) is known, by a sufficiently long waiting time since last switching on the power supply system,.
At the start of the measuring operation the measuring device activates the converter so that a predetermined converter output voltage is generated. Here the converter is preferably activated in such a way that the converter output voltage is a d-c voltage. In a further embodiment of the invention the converter output voltage is switched on only for a brief excitation interval and the switches of the converter are then actuated so that thereafter there is no further voltage present. The length of the excitation interval hereby defined should be as short as possible, for example not more than ten full oscillation cycles (although the precise length of an oscillation cycle is unknown, from the order of magnitude the length is predetermined by the components used). The preference is for a smaller number of cycles, if possible fewer than four, the particular preference even being an excitation interval of less than one cycle duration, but at least equal to one half cycle. In one possible embodiment, an estimate of the parameter sought is already obtained from the first oscillation cycle, and the result then verified by reference to the further oscillations. This is only valid, however, as long as the output voltage remains very small, that is to say a close approximation to a short-circuit can be assumed on the secondary side.
The current appearing is measured by means of the current sensor. The invention here utilizes the finding that owing to the (initially discharged) secondary-side capacitances (winding capacitance of the transformer and the smoothing capacitor) the power supply system at the instant of start-up can be regarded as short-circuited on the secondary side. Here the excitation interval is preferably greater than the observation interval, so that the resonant current can be measured without further influencing. For a measurement in the preferred short observation interval of a few oscillations, the approximation has only a small error; indeed if just the first half-oscillation is considered, a sinusoidal current path is set up with great accuracy. The path of the current is measured by the current sensor, the most important parameters of the resonant circuit, here formed from the resonant capacitance and the leakage inductance of the transformer, being measured from this path. These parameters are the resonant frequency, the impedance and the values for the resonant capacitance and resonant inductance, these four values varying as a function of one another, so that there is only a need to determine any two of the values, in order to establish the rest.
In various embodiments of the invention, different methods are proposed for determining the parameters of the power supply system that are of interest, that is the resonant frequency, the impedance and/or the values for the leakage inductance and the resonant capacitance.
The resonant frequency can easily be determined by calculating the length of time between two zero crossings of the sinusoidal resonant current occurring in the excitation. If the measurement of the current is not continuous, but merely in the form of sample values, the zero crossings can be determined by interpolation or extrapolation of sample values. The error level found here is very low owing to the relatively constant gradient of a sine function in the area of the zero crossing.
Alternatively, the measuring device can determine the resonant frequency by matching a sine function to the sample values for the current. The term xe2x80x9cmatchingxe2x80x9d is meant to imply the identification of a suitable sine function, which with regard to a degree of error approximates as closely as possible to the sampling points. A match is preferred in which the degree of error in the form of the mean square error is minimized.
Various measuring methods are also proposed for determining the impedance and the values for the resonant capacitance and inductance. The impedance can be determined with particular ease if the values for the primary-side current and the primary-side voltage are known. The primary-side voltage is predetermined by the excitation, i.e. by the converter output voltage. This is preferably constant at a known value, alternatively it can be also be measured continuously. The current path is, as stated above, to be regarded as approximately sinusoidal, at least for the first oscillation cycles. Once the excitation voltage is known, the impedance can thereby be determined very easily if the maximum value (amplitude) of the current is known.
Given a sufficient number of sample values within an oscillation cycle, the current maximum can be determined by using the maximum for the sample values. Owing to the flat characteristic of a sinusoidal curve in the area of its maximum, only a very slight error is thereby produced. According to an alternative method the amplitude of the current is calculated from sample values by matching a sine function to the sample values. This is particularly easy if the resonant frequency has already been determined by one of the aforementioned methods.
In a further embodiment of the invention the power supply system has a control device by means of which the output-side variables, such as the output voltage, can be controlled. As control variable, the control device uses the activation of the converter (e.g. the switching frequency and/or the duty cycle). Of particular importance for the design of the controller here are the previously determined parameters of the controlled system, namely the resonant frequency, impedance and/or values for resonant capacitance and inductance. The control device therefore preferably uses these previously determined parameters, so that a very precise control is possible.
This is particularly relevant, for example, in the case of X-ray units in which a highly accurate control is necessary. Stringent specifications with regard to overshoot, correction time and form of voltage run-up apply to the operation of an X-ray tube, i.e. switching on and running up the voltage to the required value. A controller adaptation enables these specifications to be met even if the components used have larger tolerances or an aging characteristic. For this purpose, for example, the prescribed measuring process can be carried out after each service interval of an X-ray unit, in order to determine the necessary parameters of the controlled system. These are then relayed to the control device where they are stored, so that in further operation the control device can operate with very precise information on the behavior of the controlled system.
The present invention provides the foregoing and other features hereinafter described and particularly pointed out in the claims. The following description and accompanying drawings set forth certain illustrative embodiments of the invention. It is to be appreciated that different embodiments of the invention may take form in various components and arrangements of components. These described embodiments being indicative of but a few of the various ways in which the principles of the invention may be employed. The drawings are only for the purpose of illustrating a preferred embodiment and are not to be construed as limiting the invention.